Method of adjusting the dimensions of sintered ferromagnetic cores

ABSTRACT

A method of adjusting the dimensions of sintered ferromagnetic coil cores by coating the cores in a mold with a layer of coating material consisting of a binder and a ferromagnetic powder.

1111 3,775,341 Dec. 4, 1973 METHOD OF ADJUSTING TIIE DIMENSIONS OF SINTERED FERROMAGNETIC CORES [75] Inventors: Franciscus Johannes Maria Lathouwers; Jacob De Groot, both Emmasingel, Eindhoven,

Netherlands [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: Sept. 29, 1971 211 App]. No.2 184,696

[30] Foreign Application Priority Data Oct. 9, 1970 Netherlands iw 7014813 52 u.s.c1 11 /605, 29/605, 29/608,

[51] Int. Cl. .i H0lf 41/02 [58] Field of Search 336/83, 233, 212;

[56] References Cited UNITED STATES PATENTS 3,325,760 6/1967 Bernard 336/83 X 476,816 6/1892 Pfannkuche 336/83 683,954 l0/l90l Liebreich........ 336/233 X 2,118,291 5/1938 Bollman 336/233 FOREIGN PATENTS OR APPLICATIONS 214,780 1924 Great Britain 336/83 Primary ExaminerThomas J. Kozma Attorney-Frank R. 'Trifari [57] ABSTRACT r A method of adjusting the dimensions of sintered ferromagnetic coil cores by coating the cores in a mold with a layerrof coating material consisting of a binder Y and a ferromagnetic powder.

2 Claims, 3 Drawing Figures FigJ - INVENTORJ FRANClSCUS' J.M.LATHOUWERS JACOB DE GROOT BY Z M Q METHOD OF ADJUSTING TIIE DIMENSIONS OF The invention will now be described with reference SINTERED FERROMAGNETIC CORES The invention relates to a method of accurately ad justing the dimensions of sintered ferromagneticcores for coils of transformersl I A treatment of this kind'is required ments are imposed as regards the shape' and the dimensions of the surface of the core, because the core is con siderably deformed'during sintejring. Such. severe requirern'ents are imposed; for example, for coils whose core has to have an accurat ely definedair gap and for cores for deflection coil a'ssemblies,the so-termed yoke rings, which have to fit accurately around the deflection coils.

According to a known method (see, for example,

Netherlands Pat. application No. 6909344), the mate-- I cated surface can be'controlled quickly and inexpen-.

sively. According to the invention gthiS. object is achieved in that after sintering, before'the core is provided with a; winding to form a coil or transformer, at least a portion of the surface of the core is coated ina mold with a layer of coating material consisting of a mixture of a binder and a ferromagnetic powder The coating material can be pressed or injected into the mold in a known. manner. The ultimate surface of the.

coated portion of the core isan accurate impression of the corresponding surface of thernould. I I

The. coating material generally-has a lower magnetic permeability-than the sintered material of the .core. This means that in coils having an air gap" whose width isto be accurately defined, also'the thickness and the permeability of the coating materialhave t'o be'taken int'oaccount'for' calculatingthe-widthof the "air gap. A

' layer of coatirig' material having a permeability .4. and

if severe requireprovide a method to the accompanying drawing, in which:

FIG. 1 is an axial sectional view of a cylindrical coil whose core is manufactured by the method according to the invention,

FIG. 2 is a partly axial sectional view of a deflection coil assembly, the yoke ringofwhich has not been treated aftersintering, and

FIG. 3 is a partly axial sectional view .ofa deflection coil assembly whose yoke ringhas been given the correct' dimensions after sintering by themethod according to the invention. a

The coil shown in FIG. 1 comprises a cylindrical winding 1 which is wound on a central cylinder 3 hav-' ing connecting wires 4. This central cylinder will generally be ferromagnetic. If the central cylinder 3 is sintered from ferromagnetic material,-the dimensions of its outer surfacecan be controlled-without much difficulty, for example, by'centerless grinding. As a result, the outer diameter of the winding 1 has a small tolerance..

Provided coaxially with thecentral cylinder 3 is a hollow,.outer cylinder 5 made of sintered ferromagnetic material, said cylinder constituting the core of the coil in co'njunction with the central cylinder 3. Between the hollow outer cylinder 5 and the winding 1 is situated an air gap 6, the width tolerance of which is determined mainly by the variations possible in the inner diameter of the'outer cylinder 5. In order to reduce this tolerance, the inner side of the outer cyIinderS is raised in a mold. by providinga layer of coating material 7,

consisting of a binder, for example,polythene, and a a thickness'd corresponds toi'an airgap having a width d/pJ. This demonstrates thatg'since the coating-layer is to compensate for the sintering'tolerances and hence has a thickness which varies for each individual product, it is desirable for proper reproducibility that the permeability of thecoating material is as large as possible. On the other hand,the quantity of binder may not be too small so as not to reduce the moldability of the coating material. A favorable compromise can .be ob-' tained by means of a coating material whose permeabilferromagnetic powder,'for example, ferrite; The layer 7 partly fills the air gap 6.- Variations in the dimensions of the outer cylinder 5 merely cause thickness variations inthe layer 7, without affecting the width of the air-filledportion 8 of. the air gap 6. The total effective width of the air gap 6 is equal to the width of the airfilled portion 8 plus the thickness of the layer 7, divided by the permeability of the coatingfmateriaLsaid permeability preferably amounting to 10;. This rneansthat a variation of 0.1 .mm inthe-thickness of layer ,7, caused by the tolerance of the =l1ollow. outer cylinder 5, corresponds to a'variation. of"only 0.0l .mm. inthe total'effective width of the air gap 6. 1

tween the central cylinder 3 and the outer cylinder '5 each of the connecting wires 4 can be secured-in'the (hollow) central cylinder by means of a plug 9 of the same material as the layer 7. Th e plug 9 can be conity has a value of approximately 10 after hardening of the'binder. Thickness variations in the coating material tion of the plug 9 extending outsidethe central cylinthen correspond to 'width variations of the air gap whichfare 10 times as small, which is in view of the usual tolerances of sintered cores fullyacceptable for most applications. r i It is to be noted that the surrounding of a coil bya moulding compound is known, for example, from Swiss Pat. Spec; No. 228,763. The windings of the coil, however, are then embedded in 'the moulding compound, after'they have been provided on the core, so as to increase the inductance, thus forming an integral unit. A

v method of this kind,-consequently-, cannot be used, for

example, for giving a yoke ring the correct shape or for providing a coil core with an accurately defined air gap.

nected to the'outer cylinder 5 by means of a disc 10 which is also formed from the same material.

Providing the winding 1 on the central cylinder 3 can be facilitated by imparting a conical surface to the porj der, so that a gradual change-over from the diameter of of saddle coils 11 for the frame deflection (the pair of line deflection coils provided within the frame coils is omitted for the sake of clarity). The coil pair 11 is surrounded by a yoke ring-13 of sintered ferrite, having a flared inner surface which has not been treated after sintering. The Figure clearly illustrates that a yoke ring 13 which has shrunk only slightly more than the ring 13, will take in a position which has shifted considerably in the axial direction. I

This drawback does not occur with the yoke ring shown in FIG. 3 and surrounding a similar coil assembly as in FIG. 2. The flared inner side of the yoke ring 15 is raised in a mould by coating with a layer of coating material 17 of the same composition as the layer 7 in FIG. 1. Consequently, the inner surface of the yoke ring 15 always adjoins the comparatively accurately reproducible outer surface of the coil pair 11 in the same manner, so that the yoke ring 15 will always take in the same position with respect to this coil pair in the axial direction.

mately 10. 

1,. A method of accurately adjusting the dimensions of a sintered ferromagnetic core for a coil or transformer, comprising the steps of, placing the core in a mold, applying to at least a portion of the surface of the core a layer of coating material consisting of a mixture of a binder and a ferromagnetic powder while in said mold, and thereafter winding a coil on said core. 2,. A method as claimed in claim 1, wherein the magnetic permeability of the coating material is approximately
 10. 